INFRARED AND RAMAN-SPECTRA, CONFORMATIONAL STABILITY, AB-INITIO CALCULATIONS, AND VIBRATIONAL ASSIGNMENTS FOR CYCLOPROPYLCHLOROSILANE

The infrared (3300-30 cm(-1)) spectra of gaseous and solid cyclopropyl chlorosilane, c-C3H5SiH2Cl, have bees recorded. Additionally, the Rama n spectra (3200-30 cm(-1)) of the liquid and solid have been recorded and quantitative depolarization values obtained. Both the cis-and gauc he conformers have been identified in the fluid phases but only the ga uche conformer remains in the solid. Variable-temperature (-55 to -100 degrees C) studies of the infrared spectra of the sample dissolved in liquid xenon have been carried out. From these data, the enthalpy dif ference has been determined to be 98 +/- 10 cm(-1) (280 +/- 29 cal mol (-1)), but with the cis conformer being the more stable form which is consistent with the predictions from ab initio calculations at the hig hest level of calculation, MP2/6-311++G*. A complete vibrational assi gnment is proposed for both the cis and gauche conformers based on inf rared band contours, relative intensities, depolarization values, and group frequencies. The vibrational assignments are supported by normal -coordinate calculations utilizing the force constants from ab initio MP2/6-31G calculations. Complete equilibrium geometries have been det ermined for both rotamers by ab initio calculations employing a variet y of basis sets up to 6-311++G* at levels of restricted Hartree-Fock (RHF) and/or Moller-Plesset (MP) to second order. The potential energy terms for the conformer interconversion have been obtained from the M P2/6-31G calculation. The results are discussed and compared to those obtained for some similar molecules.